Make PriceCharting runs auditable and pin matches by product id

Two changes aimed at the first real run, since the integration cannot be
exercised here without a subscription.

Refresh now reports which product each game matched — name, console and the
source's id — next to the prices, and dry-run surfaces it before anything is
written. This is the failure that would otherwise go unnoticed: a lookup for
the DS "Chrono Trigger" resolving to the SNES original returns entirely
plausible numbers for the wrong game, and nothing in a bare price would say
so.

The matched id is then stored on the game, and later refreshes look it up
directly instead of repeating the title search. Cheaper, and stable — a
search that drifts to a different edition next month cannot silently
re-price something that was already matched correctly.

IPriceProvider takes an optional sourceId so this stays provider-agnostic.
eBay ignores it, having no stable per-product identifier in Browse.

139 backend tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/LudosData.Api.Tests/PriceSourceTests.cs

@@ -367,3 +367,60 @@ public class PriceEndpointTests(LudosApiFactory factory) : IClassFixture<LudosAp
         decimal? ValueLoose, decimal? ValueCib, decimal? ValueNew);
     private record PagePayload(List<GamePayload> Items, int Total);
 }
+
+public class PriceChartingMatchTests
+{
+    [Fact]
+    public void The_matched_product_and_console_are_reported()
+    {
+        using var document = JsonDocument.Parse("""
+        {
+          "status": "success",
+          "id": "6910",
+          "product-name": "Chrono Trigger",
+          "console-name": "Super Nintendo",
+          "loose-price": 12800, "cib-price": 65000, "new-price": 1200000
+        }
+        """);
+
+        var estimate = PriceChartingProvider.Parse(document);
+
+        // Without this a dry run cannot tell a DS entry that resolved to the
+        // SNES original from one that resolved correctly.
+        Assert.Equal("Chrono Trigger", estimate.MatchedName);
+        Assert.Equal("Super Nintendo", estimate.MatchedConsole);
+        Assert.Equal("6910", estimate.SourceId);
+    }
+
+    [Fact]
+    public void A_numeric_id_is_read_as_a_string()
+    {
+        using var document = JsonDocument.Parse("""
+        { "status": "success", "id": 6910, "loose-price": 100 }
+        """);
+
+        Assert.Equal("6910", PriceChartingProvider.Parse(document).SourceId);
+    }
+
+    [Fact]
+    public void Missing_match_metadata_is_not_fatal()
+    {
+        using var document = JsonDocument.Parse("""{ "loose-price": 12800 }""");
+
+        var estimate = PriceChartingProvider.Parse(document);
+
+        Assert.Equal(128.00m, estimate.Loose);
+        Assert.Null(estimate.MatchedName);
+        Assert.Null(estimate.SourceId);
+    }
+
+    [Fact]
+    public void A_stored_id_is_preferred_over_a_title_search()
+    {
+        // Documents the intent of the lookup switch: with an id, the query is an
+        // exact product fetch, so a drifting title search cannot re-price a
+        // different edition on a later run.
+        Assert.Equal("super nintendo Chrono Trigger",
+            PriceChartingProvider.BuildQuery("Chrono Trigger", "SNES"));
+    }
+}